Radio scanning is typically an option included in a two-way radio that enables the user to rapidly receive or scan multiple frequencies in a predetermined order. This enables more than one frequency or channel to be monitored for incoming voice or data enabling the two-way radio to be versatile.
It is highly desirable for two-way radio equipment that utilizes a conventional scanning technique to scan radio channels at the fastest rate possible. This avoids lost syllables during speech and prevents "audio holes" or gaps of speech from being missed. If the radio must decode subaudible signaling, scan performance becomes degraded whenever "unqualified" channel activity is detected because the radio must spend up to 200 ms decoding the signaling. This problem is even further exacerbated when the radio operates primarily on a fixed set of itinerant or consumer channels. These channels are highly utilized especially in urban or industrial environments, and this increases the chance of scan-degrading unqualified channel activity. With regard to most commonly used conventional scanning techniques, these techniques operate by iteratively checking each channel in a list for a carrier.
As seen in prior art FIG. 1, the scanning sequence starts 101 where the receiver is set up on each specific channel 103. If no radio frequency (RF) carrier 105 is detected, the next channel is selected 107 and the receiver is configured for the next channel 103 in the list. Channels are scanned iteratively 103, 105, 107 from a list until a carrier is detected. Upon completion of scanning the list, the list is reset and scanning continues from the top of the list. If an RF carrier signal is detected 105, the receiver checks for a valid subaudible code 109 such as a private line (PL) or digital private line (DPL) multi-frequency tone. If no subaudible code is detected 111 the process begins again by selecting the next channel to scan 107. In the event that the proper subaudible squelch code is detected 111, the receiver is unmuted 113 and the proper information is received until no further RF carrier signal or subaudible code is present. After no further RF carrier signal or subaudible is present, the next channel in the scan sequence is selected 107.
Additionally in prior art FIG. 2, another conventional scan technique utilizes subaudible squelch coding and channel marking 200. Channel marking is used to "mark" a channel with an RF carrier and a non-matching subaudible squelch code, so future scans of a marked channel will not incur the delay associated with decoding the subaudible signaling. A loss of the RF carrier on a marked channel will cause the channel to become "unmarked," whereby full decoding will take place followed by a decision whether to receive the information. A counter for each channel keeps track of how many times the channel is "skipped" if it is marked, and if a limit is reached, that particular channel is unmarked. Those skilled in the art will appreciate the improved scan performance associated with this technique. In this technique, the scanning sequence starts 201 by setting up the receiver on the specific receiving channel 203. If no RF carrier 205 is detected the radio checks a list to determine if it is marked 207. If the channel is not marked. the next subsequent channel in the scan sequence is selected 209. If however, the channel is marked the proper steps are taken to unmark the channel 211 and clear the marked channel counter.
If an RF carrier is detected 205, a determination is again made as to whether the channel is marked 213. If the channel is marked a channel marking counter is incremented 215 for that channel and a comparison is made 217 to determine if a limit has been reached. If no mark limit has been reached the next channel in the scanning sequence 209 is selected. If however, the mark limit has been reached the mark and mark limit counter is cleared 221 and any associated signaling information is subsequently decoded 219. Thereafter, if no signaling information 223 is detected the channel is marked 225 and the next channel is selected in the scan sequence 209. If signaling information is detected, the data is received or the receiver is unmuted 227 until no further channel activity is detected 229.
If a carrier is detected, the radio will pause on that channel and will attempt to decode the private line (PL), digital private line (DPL) code or other signaling information. If the correct detection is made, the message will be received or the receiver unmuted; otherwise, the radio will resume scanning. If there are large quantities of "unqualified" activity, that is channel activity with a missing or non-matching signaling code, scan performance will seriously be degraded. This is due to the fact that the radio must continually stop on each channel and perform the time-consuming task of checking for subaudible signaling information. Previous radio scanning techniques have improved performance by "marking" a specific channel that has unqualified activity. This ultimately works to prevent unnecessary PL decodes. The scanning controller or "executive" simply checks the marked channel for loss of a carrier. When such a transition is detected, the channel is unmarked, so additional activity receives a full decode attempt.
Users of radios on business frequencies often work in small groups within a confined geographical area, such as a warehouse. It is not uncommon for such a group of people to be within the receiving range of another group legally assigned the same frequency. Typically the two groups will not interfere with each other due to selection of different subaudible squelch codes, but such interference can degrade scan performance, even with channel marking. Channel activity from another group using a different code will cause a scanning radio to mark a channel, which will cause a latency in receiving a valid transmission. The worst case involves the receipt of a valid message just after a channel has been marked from a user of another group. The valid message will not be received until the mark limit counter reaches its limit, which forces a decode. If the message is short, the message may not be received at all. Reducing the mark limit counter will reduce the latency, but will degrade scan performance.
Thus the need exists to provide a radio scanning method that utilizes a channel marking scheme for scanning efficiency but provides a means to minimize message latency on channels likely to have an abundance of unqualified activity.